1. Field of the Inventions
Embodiments of the present invention generally relate to the storage of large fluid volumes. More particularly, embodiments of the present invention relate to tank designs for holding hydrocarbons. In addition, embodiments of the present invention relate to the manufacture of an LNG containment system.
2. Description of Related Art
Clean burning natural gas has become the fuel of choice in many commercial and consumer markets around the industrial world. Such natural gas is oftentimes transported across oceans from the sites of production to consuming nations. Such transportation of natural gas typically occurs over long distances using large-volume marine vessels.
In order to facilitate transportation the gas is taken through a liquefaction process. The liquefied natural gas, or “LNG”, is formed by chilling very light hydrocarbons, e.g., hydrocarbons comprised primarily of methane, to approximately −163° C., where it is stored at ambient pressure in special cryogenic tanks. Due to its low critical temperature, continued refrigeration is desired for LNG transportation and storage.
Upon delivery to an import terminal, the LNG is typically stored for later use and delivery to domestic markets. Experience shows that bulk storage of liquefied natural gas is most economical when stored in its fully refrigerated state, and at its bubble point at or near atmospheric pressure. The boiling point of LNG at one atmosphere is approximately −163° C. To accommodate this condition, insulated storage tanks are employed. The LNG storage tanks typically have a primary container and a surrounding secondary container.
For large volume storage of LNG, two distinct types of tank construction are widely used. The first of these is a flat-bottomed, cylindrical, self-standing tank that typically uses a 9% nickel steel for the inner tank and carbon steel, 9% nickel steel, or reinforced/prestressed concrete for the outer tank. The second type is a membrane tank wherein a thin (e.g. 1.2 mm thick) metallic membrane is installed within a cylindrical concrete structure which, in turn, is built either below or above grade on land. A layer of insulation is typically interposed between the metallic membrane, e.g., of stainless steel, and the load bearing concrete cylindrical walls and flat floor.
In the context of the cylindrical, self-standing LNG tank, and from a safety and environmental standpoint, it is preferred that the tank have “full containment.” A “full containment” system requires that the outer secondary container hold both liquid and its vapor should the liquid escape from the primary container. The full containment system should also be configured to permit the controlled release or withdrawal of these fluid products from the system. While structurally efficient, cylindrical tanks in their state-of-practice designs are difficult and time consuming to build. LNG storage systems using self-standing 9% nickel steel tanks may require up to 36 months for construction. On many projects, this causes undesirable escalation of construction costs and length of construction schedule.
A need exists for a full containment LNG storage system that provides liquid and vapor integrity in the event of primary container leakage, and that can be efficiently fabricated. A need further exists for an improved method of fabricating a secondary container, such as an LNG container. A need further exists for prefabricated wall and roof panels that may be brought to a construction site for efficient erection of secondary container walls and roof structure.